Acotyledon is used to refer to seed plants or spermatophytes that lack cotyledons, such as orchids and dodder. Orchid seeds are tiny with underdeveloped embryos. They depend on mycorrhizal fungi for their early nutrition so are myco-heterotrophs at that stage.

Contrary to many websites, acotyledon is not used to refer to "all nonflowering plants." Gymnosperms are nonflowering plants that have cotyledons or seed leaves. Seedless, nonflowering plants, such as bryophytes, ferns and fern allies, are not referred to as acotyledons because they lack seeds altogether.

Flowering plants or angiosperms are divided into two large groups. Monocotyledons or monocots have one seed lobe, which is often modified to absorb stored nutrients from the seed so never emerges from the seed or becomes photosynthetic. Dicotyledons or dicots have two cotyledons and often germinate to produce two leaf-like cotyledons. Conifers and other gymnosperms lack flowers but may have two or more cotyledons in the seedling.